"Mass incarceration means that this country approaches its problems through the criminal-justice system. When faced with a social ill, our nation responds by building more prisons and jails. Because incarceration is the tool that we use to address societal problems, we have erected few limitations on the police’s ability to keep the social order. Police can stop whomever they want to stop whenever they want to stop them. They can investigate things that have no relation to the reason for the stop. They can use force. They can kill."